Hitherto, polymethyl methacrylate (PMMA) and polycarbonate (PC) have been known as optical polymer materials. However, PMMA is excellent in transparency, but has problems in heat resistance and moisture resistance. PC is superior to PMMA in heat resistance and moisture resistance, but has the problem that it is apt to have a high birefringence due to its basic structure having a benzene ring and its high melt viscosity.
Recently, ring-opening polymerization polymer hydrogenation products of norbornene monomers are used as polymer materials improved in these defects. These resins are excellent in heat resistance and moisture resistance and, furthermore, have low birefringence. However, resins of further lower birefringence are required by the further advancing technical demand.
1,4-Methano-1,4,4a,9a-tetrahydrofluorene (hereinafter referred to as "MTF") is a monomer disclosed as an example of the norbornene monomers (e.g., JP-A-3-273043, etc.). However, no examples of ring-opening polymerizing the monomer and hydrogenating the polymer are disclosed in the prior art. Therefore, no characteristics have been known on the ring-opening polymerization polymer hydrogenation products of MTF. Furthermore, hydrogenation products of ring-opening polymerization copolymers of 1,4-methano-1,4,4a,4b,5,8,8a,9a-octahydro-9H-fluorene having the similar structure to MTF (differing from MTF in that the aromatic ring structure of MTF is replaced with a cyclohexene ring structure) and 5,8-methano-3a,4a,5,8,8a,9,9a-octahydro-1H-benzoindene at a molar ratio of 4:6-6:4 are known (JP-A-3-220230). The resins have a small birefringence and have a strength and a heat resistance. However, they do not have such level of low birefringence as newly required.
Moreover, the norbornene ring-opening polymerization polymer hydrogenation products suffer from the problem that they sometimes undergo reduction in mechanical strength when greases and oils stick thereto to cause breakage.